Render window keeps popping up with the last frame of recent render
I am also relatively new to blender but enjoying it ALOT. 

As for your problem, in my opinion, this is because the particular render is sort of 'attached' to your active camera which i assume , that you used with keyframe insertions in your animation timeline. To circumvent this, I usually create a new camera and select it to be the 'active' (see below). Then F12 as usual to render from that camera.